Today, propagation of solar batteries is tremendous because of recent severe energy conditions. As well known, the solar battery is a device that converts sunlight energy into electric power. The sunlight energy amount changes every second due to shadows of clouds etc. and rotation of the earth. Therefore, the maximum output power that can be generated the solar battery changes in correspondence with the changing sunlight energy amount reaching the solar battery. Thus, there has been proposed and known maximum power point tracking control (MDPT) for performing control such that the power generatable by a solar battery at a given time point can be output to a load with a maximum output efficiency.
Also known today is a technique for detecting a maximum output power point by means of a so-called “hill climbing” method in order to realize such MPPT control, as disclosed for example in Japanese Patent Application Laid-Open Publication No. HEI-09-56180. The “hill climbing” method comprises increasing or decreasing the output of the solar battery by controlling the duty cycle of a solar-battery output controlling switching element at a given control frequency. Namely, with respect to power of the solar battery output at a time point, the duty cycle of the solar-battery output controlling switching element is increased or decreased by a given variation width in the next or subsequent control period.
More specifically, according to the “hill climbing” method, if the output power of the solar battery has increased in response to increase or decrease of the duty cycle of the solar-battery output controlling switching element, it is determined that the maximum output power point of the solar battery is located in a positive direction as viewed from power of the solar battery detected at a current time point; on the other hand, if the output from the solar battery has decreased in response to increase or decrease of the duty cycle, it is determined that the maximum output power point is located in a negative direction as viewed from power of the solar battery detected at a current time point. In the case where it has been determined that the maximum output power point of the solar battery is located in the positive direction as viewed from the power of the solar battery detected at the current time point, the duty cycle at the current time point is increased by a given value and then the duty cycle is increased or decreased again in the next control period, and then a further determination is made as to whether the power output from the solar battery in response to the thus-increased or decreased duty cycle is in the positive direction or in the negative direction as viewed from the maximum output power point. The aforementioned operations are repeated so as to control the actual power output from the solar battery to approach the maximum output power point.
The output power control method of the solar battery is performed by a power conditioner (i.e., maximum power point tracking control apparatus) increasing or decreasing the duty cycle of a solar-battery output power controlling switching element of a DC-DC converter. At that time, the output power control method constantly controls the duty cycle little by little, for example, by increasing or decreasing the duty cycle in a sine-wave manner while checking how the output power of the solar battery changes in response to increase or decrease of the duty cycle, so that the output power of the solar battery can become maximum. However, if the output power of the solar battery has changed greatly, the aforementioned approach of increasing or decreasing the duty cycle little by little would require a non-negligible, considerable time before the maximum output power point of the solar battery can be found or tracked. This means that a considerable time would be required before the power conditioner outputs the maximum power, generatable by the solar battery at a given time point, to a load; namely, during that time, the power conditioner can not supply the maximum power, generatable by the solar battery, to the load. Therefore, the conventionally-known technique presents the problem that output efficiency of the solar battery, indicative of how much of sunlight energy could be converted into electric power, would unavoidably decrease.